An international team of astronomers has used the NASA/ESA Hubble Space Telescope to look for atmospheres around four Earth-sized planets orbiting within or near TRAPPIST-1’s habitable zone. The new results further support the terrestrial and potentially habitable nature of three of the studied planets. The results are published in Nature Astronomy.

SulutPos.com, Garching bei München, Germany – Seven Earth-sized planets orbit the ultracool dwarf star TRAPPIST-1, 40 light-years away from the Earth [1]. This makes TRAPPIST-1 the planetary system with the largest number of Earth-sized planets discovered so far. These planets are also relatively temperate, making them a tantalizing place to search for signs of life beyond our Solar System. Now, an international team of astronomers has presented a study in which they used the NASA/ESA Hubble Space Telescope to screen four planets in the system — TRAPPIST-1d, e, f and g — to study their atmospheres [2].

Comparing the TRAPPIST-1 planets
. A size comparison of the planets of the TRAPPIST-1 system, lined up in order of increasing distance from their host star. The planetary surfaces are portrayed with an artist’s impression of their potential surface features, including water, ice, and atmospheres.
Credit:
NASA/R. Hurt/T. Pyle

Seven planets orbiting the ultracool dwarf star TRAPPIST-1. This is an artist’s impression of the TRAPPIST-1 system, showcasing all seven planets in various phases. When a planet transits across the disk of the red dwarf host star, as two of the planets here are shown to do, it creates a dip in the star’s light that can be detected from Earth.
Also during such transits astronomers are able to study the potential atmospheres of these planets.
Credit:
NASA

Artist’s illustrations of planets in TRAPPIST-1 system and Solar System rocky planets. This infographic displays some artist’s illustrations of how the seven planets orbiting TRAPPIST-1 might appear — including the possible presence of water oceans — alongside some images of the rocky planets in our Solar System. Information about the size and orbital periods of all the planets is also provided for comparison; the TRAPPIST-1 planets are all approximately Earth-sized.
Credit:
NASA

Three of the planets orbit within the system’s habitable zone, the region at a distance from the star where liquid water — the key to life as we know it — could exist on the surface of a planet. The fourth planet orbits in a borderline region at the inner edge of the habitable zone. The data obtained rule out a cloud-free hydrogen-rich atmosphere for three of the planets — but for the fourth planet, TRAPPIST-1g, such an atmosphere could not be excluded [3].

Lead author Julien de Wit, from the Massachusetts Institute of Technology, USA, describes the positive implications of these measurements: “The presence of puffy, hydrogen-dominated atmospheres would have indicated that these planets are more likely gaseous worlds like Neptune. The lack of hydrogen in their atmospheres further supports theories about the planets being terrestrial in nature. This discovery is an important step towards determining if the planets might harbour liquid water on their surfaces, which could enable them to support living organisms.”

The observations were made while the planets were in transit in front of TRAPPIST-1. In this configuration a small section of the star’s light passes through the atmosphere of the exoplanet and interacts with the atoms and molecules in it. This leaves a weak fingerprint of the atmosphere in the spectrum of the star.

The habitable zone in the TRAPPIST-1 system. The TRAPPIST-1 system contains a total of seven known Earth-sized planets. Three of them — TRAPPIST-1e, f and g — are located in the habitable zone of the star (shown in green in this artist’s impression), where temperatures are just right for liquid water to exist on the surface.
While TRAPPIST-1b, c and d are too close to their parent star and TRAPPIST-1h is too far away, the remaining three planets could have the right conditions to harbour life.
As a comparison to the TRAPPIST-1 system the inner part of the Solar System and its habitable zone is shown.
Credit:
NASA/JPL-Caltech

Spectra of planets in TRAPPIST-1 system. These spectra show the chemical makeup of the atmospheres of four of the Earth-sized planets orbiting within or near the habitable zone of the star TRAPPIST-1.
To obtain the spectra, astronomers used the NASA/ESA Hubble Space Telescope to collect light from TRAPPIST-1 passing through the exoplanets’ atmospheres as the exoplanets crossed in front of the star.
The purple curves show the predicted signatures of gases such as water and methane that absorb certain wavelengths of light. These gases would be found in a puffy hydrogen-dominated atmosphere similar to those of gaseous planets such as Neptune. The Hubble results, indicated by the green crosses, reveal no evidence of an extended atmosphere in three of the exoplanets (TRAPPIST-1d, f, and e). Additional observations are needed to rule out a hydrogen-dominated atmosphere for the fourth planet (TRAPPIST-1g).
The evidence indicates that the atmospheres are more compact than could be measured by the Hubble observations.
Credit:
NASA, ESA and Z. Levy (STScI)

Starlight yields clues to exoplanets’ atmospheres. Astronomers used the NASA/ESA Hubble Space Telescope to analyse light from the nearby star TRAPPIST-1 as it passed through the atmospheres of four Earth-sized planets in the star’s habitable zone — the region at a distance from the star where liquid water, the key to life as we know it, could exist on their surfaces. The astronomers were looking for the signatures of certain gases, including hydrogen, in the atmospheres that were imprinted on the starlight.
The graphic at the top shows a model spectrum containing the signatures of gases that the astronomers would expect to see if the exoplanets’ atmospheres were puffy and dominated by primordial hydrogen from the distant worlds’ formation.
The Hubble observations, however, revealed that the planets do not have hydrogen-dominated atmospheres. The flatter spectrum shown in the lower illustration indicates that Hubble did not spot any traces of water or methane, which are abundant in hydrogen-rich atmospheres. The researchers concluded that the atmospheres are composed of heavier elements residing at much lower altitudes than could be measured by the Hubble observations.
Credit:
NASA, ESA and Z. Levy (STScI)

While the results rule out one type of atmosphere, many alternative atmospheric scenarios are still consistent with the data gathered by de Wit and his team. The exoplanets may possess a range of atmospheres, just like the terrestrial planets in our Solar System [4].

“Our results demonstrate Hubble’s ability to study the atmospheres of Earth-sized planets. But the telescope is really working at the limit of what it can do,” adds co-author Hannah Wakeford from the Space Telescope Science Institute, illustrating both the power and limitation of Hubble.

These latest findings complement the analysis of ultraviolet observations made with Hubble in 2017 (heic1713) and help us understand more about whether life might be possible in the TRAPPIST-1 system.

“Spectroscopic observations of the TRAPPIST-1 planets with the next generation of telescopes — including the James Webb Space Telescope — will allow us to probe deeper into their atmospheres,” concludes Michael Gillon, from the University of Líege, Belgium. “This will allow us to search for heavier gases such as carbon, methane, water, and oxygen, which could offer biosignatures for life.”

[3] An atmosphere largely dominated by hydrogen, if cloud-free, should yield prominent spectroscopic signatures in the near infrared. However, the spectra for TRAPPIST-1d, -e, and -f do not show significant features.

[4] This includes atmospheres dominated by water vapor, nitrogen, carbon dioxide or tenuous atmospheres composed of a variety of chemical species.

More information

The Hubble Space Telescope is a project of international cooperation between ESA and NASA.